This invention relates generally to a mold assembly and method for pressure casting elevated melting temperature alloys and pressure infiltration casting metal matrix composite structures, and more particularly to such a mold assembly having both ceramic and metal components and to a method of hybrid casting using a mold assembly having both ceramic and metal components.
Pressure casting, also commonly referred to as squeeze casting, has long been advocated as the ideal process for the production of metal matrix composite (MMC) castings, and as a method of eliminating porosity in cast alloys. However, heretofore pressure casting of liquid metal alloys has been generally limited to relatively low melting temperature alloys, such as aluminum. A common problem when casting relatively higher melting temperature alloys has been the tendency of the higher melting temperature alloys to at least partially bond, i.e., weld, to the surface of a metal die in which the higher melting temperature alloy is cast.
An example of pressure casting of relatively low melting temperature metal alloys is described in U.S. Pat. No. 5,511,603 issued Apr. 30, 1996 to Alexander M. Brown, et al and entitled MACHINABLE METAL-MATRIX COMPOSITE AND LIQUID METAL INFILTRATION PROCESS FOR MAKING SAME. In the Brown, et al process, metal matrix composites are formed by pressure casting in which the pressure is supplied by an inert gas, such as argon, and the structure is cast into a previously evacuated ceramic mold. The ceramic mold was coated with a graphite coating and then lined with graphite paper prior to heating and casting of the molten metal.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a mold assembly, and method of casting, in which relatively high, i.e., elevated melting temperature alloys, can be cast. It is also desirable to have such a mold assembly and method that does not require the application of special coatings and linings to the mold, which could disadvantageously modify the surface chemistry of elevated melting temperature materials. It is also desirable to have a mold assembly and method of casting that does not require a pressurized inert gas atmosphere. Moreover, it is desirable to have a mold assembly and method of squeeze, or pressure, casting of elevated melting temperature alloys, both ferrous and nonferrous, which provide porosity-free, near net-shape cast components. In addition, it is desirable to have a mold assembly and method of pressure infiltration casting of elevated melting temperature alloys, both ferrous and nonferrous, into loose or loosely held ceramic or cermet particles, porous preforms made of ceramic or cermet powders, and monolithic preforms made of ceramic or cermet powders.
In one aspect of the present invention, a mold assembly suitable for pressure infiltration casting elevated melting temperature alloys and metal matrix composite structures includes a liquid metal impermeable ceramic mold disposed within a steel die. The liquid metal impermeable ceramic mold has an inner surface that defines the external shape of an article cast in the ceramic mold, and an outer shape that substantially conforms to the inner surface of a steel die. Ideally the wall thickness of the ceramic mold would be from about 2 mm to about 6 mm. The ceramic mold also has an opening formed in an upper portion that is adapted to receive a pressure-actuated punch therein having a ceramic cap disposed on a distal end of the punch. The steel die has an inner surface which encloses and mates with the outer surface of the ceramic mold whereby the steel die intimately supports the ceramic mold within the internal cavity.
In another aspect of the present invention, a mold assembly for pressure casting elevated temperature metal alloys and metal matrix composite structures, includes a liquid metal impermeable ceramic mold, as defined above in the previously described aspect of the present invention, a granular support media surrounding the ceramic mold in intimate contact with the outer surface of the ceramic mold, and a steel die. The steel die has an inner surface defining an internal cavity shaped to support the granular support media therein and has an opening formed in an upper portion adapted to receive a low pressure punch reciprocatably movable between the outer surface of the ceramic mold and the inner surface of the steel die whereby the granular support media is maintained in a compressed state within the internal cavity of the steel die.
Other aspects of the present invention include the punch received through the opening in the upper portion of the ceramic mold having a ceramic cap disposed on a distal end of the punch. Another feature of the mold assembly embodying the present invention includes the granular support media being either metallic or non-metallic particles.
In another aspect of the present invention, a method of forming porosity-free, near net-shape articles containing elevated melting temperature alloys includes providing a liquid metal impermeable ceramic mold having a wall thickness from about 2 mm to about 6 mm and an opening disposed in a top portion adapted to receive a punch member therethrough, heating the ceramic mold to a temperature substantially equal to 1000xc2x0 C. (1832xc2x0 F.), and providing an alloy steel die having an internal cavity adapted to receive the ceramic mold therein. The method further includes heating the alloy steel die to a temperature substantially equal to 260xc2x0 C. (500xc2x0 F.), inserting the heated ceramic mold into the internal cavity of the heated alloy steel die, and pouring a molten elevated melting temperature metal into the ceramic mold. The method then includes lowering the punch member through the opening in the upper portion of the ceramic mold, thereby bringing the punch member into intimate contact with the molten metal poured into the ceramic mold. The lowering of the punch is continued so as to create pressure on the molten metal sufficient to form an essentially porosity-free article having a net shape defined by the internal surface of the ceramic mold. The alloy steel die, the ceramic mold and the metal cast in the mold is then cooled, thereby forming a solidified cast article in the mold, after which the solidified cast article is removed from the mold.
Other features of the method of forming porosity-free, near net-shape articles, includes inserting a wear-resistant material into the mold prior to pouring a molten elevated melting temperature metal into the mold. Another feature includes preheating the wear-resistant insert or preform, prior to inserting the wear-resistant material into the mold. Other features include the wear-resistant material comprising wear-resistant fibers, wear-resistant particles, or a preformed monolithic article, having either a porous or solid structure.
Still other features of the method of forming porosity-free, near net shape article, in accordance with the present invention, includes pouring a molten elevated melting temperature metal into the ceramic mold in which the metal has a melting temperature of at least 900xc2x0 C. (1652xc2x0 F.). Examples of such elevated melting temperature metals includes gray iron and low alloy steel.
Yet another feature of the method of forming porosity-free, near net-shaped articles, in accordance with the present invention, includes the step of cooling the alloy steel die, the ceramic mold, and the metal alloy cast in the mold in such a manner as that the first cooled portions of the die, mold, and cast metal alloy, are spaced furthest from the punch member. The die, mold, and cast alloy are then sequentially cooled from the portions first cooled toward an interface between the cast metal alloy and the punch member, thereby causing directional solidification of the cast article.